Riding mowers are well known for use in cutting grass in relatively wide swaths over a large area. Hydraulic drive systems powered by an internal combustion engine or other power source carried on the mower are also well known for propelling the mower in forward or reverse. Such drive systems often comprise individual hydraulic motors that engage each of the ground engaging wheels of the mower such that every wheel on the motor is a drive wheel. For example, mowers are known having four ground engaging wheels powered by four hydraulic motors to provide four wheel drive (4WD).
Different hydraulic fluid supply circuits can be used to power a 4WD system. One common system is an all parallel circuit in which the hydraulic motors are all connected in parallel to the hydraulic fluid source, typically a single hydraulic pump. Other systems comprise various combinations of series-parallel circuits in which some motors are connected in parallel and others are connected in series.
Each of these systems has its own advantages and disadvantages. Parallel connections provide an inherent differential effect since the motors driving the wheels on the outside of a turn will receive more flow and can rotate faster than the motors driving the wheels on the inside of the turn. Series connections can enhance traction in certain circumstances, but lose the inherent differential effect provided by a parallel connection. Series connections for mowers must be provided with some way to vary the flow passing through the motors on the insides and outsides of a turn, or alternatively to allow the wheels to overrun the motors, to avoid scrubbing of the wheels during a turn.
Various ways have been proposed to allow such flow variation in hydraulic circuits that incorporate series connections between the motors. Actively controlled valves can be used to port more flow to the motors for the wheels on the outsides of the turn as compared to motors for the wheels on the insides of the turn. However, this requires such valves to be present along with a control system for operating the valves as the mower turns. Overrunning clutches and or overrunning hydraulic check valves can be used to allow the wheels to overrun the speed of rotation of the motor shafts when necessary.
Most of the solutions known in the art for providing flow variation in circuits having series connections have been developed for use when only a single pair of the wheels on the mower, typically the front wheels, are steered. The wheels in the non-steered pair of wheels, typically the rear wheels, do not follow dramatically different paths about the center of the turn, and so the flow variation needed for the motors driving the non-steered wheels is not that extreme. However, in a mower with a pair of steerable front wheels and a pair of non-steered rear wheels, the turning radius of the mower is relatively large. Thus, it can be difficult to maneuver the mower in tight spaces without having the deck hit an obstruction, such as a wall or a tree, which the operator is attempting to mow around. Typically, the operator can get close to the obstruction, but may need to leave an uncut strip or swath of grass next to the obstruction in order to avoid hitting it.
It would be an advance in the art to provide a riding mower with four wheel steering (4WS) to shorten the turning radius and have the center of the turn be located at approximately the outer lateral edge of the cutting deck that is adjacent the obstruction. This would permit close cutting up to an obstruction without hitting the obstruction and without leaving any uncut strips of grass. However, in such a 4WS mower with a hydraulic 4WD system, the Applicants have discovered that the demand for different amounts of fluid in the motors driving the various wheels is greatly exacerbated in tight turns. In other words, the Applicants have found that the variation in the amounts of fluid needed by the various motors driving the various wheels greatly increases over the variations normally present in a two wheel steer mower. This invention is directed to providing such a mower, namely a riding mower having 4WS and a hydraulic 4WD system that incorporates at least some series connections between the motors, which can accommodate the necessary flow variations to the motors in a simple and cost-effective manner.